The metals in question are preferably those which yield so-called hard substances, such as titanium, zirconium tantalum, vanadium and hafnium, for example. The hard substances are compounds of at least one of these metals with nitrogen (nitrides), carbon (carbides) and with carbon and nitrogen simultaneously (so-called carbonitrides). The method, however, is also suitable for the reactive vapor deposition of other metals, which for example are reacted with oxygen to oxides.
A method of the kind described above is disclosed in the article by Bunshah and Raghuram, entitled "Activated Reactive Evaporation Process for High Rate Deposition of Compounds" published in J. Vac. Sci. Technol., vol. 9, No. 6, November-December 1972, pages 1385 to 1388. In the method therein described, the metal is vaporized directly in the vacuum chamber by means of an electron beam of relatively low acceleration voltage (10 kV). Likewise, a wire-like electrode biased positively with respect to ground, e.g., to a voltage between 80 and 200 volts, is situated directly in the vapor stream. In the operation of this apparatus, the electron beam serves not only for the evaporation of the metal, but also as an electron donor, while the positive electrode attracts negative charge carriers, thereby increasing the ionization probability, and is surrounded directly by a glow discharge. The known method and apparatus, however, have not progressed beyond their use on a laboratory scale, since the evaporation rate cannot be increased to the desired level for large-scale industrial processes. All attempts to increase the evaporation rate by increasing the beam current and hence the beam power have failed because the reaction no longer was stoichiometric, even though the nozzle for the injection of the reaction gas discharges directly above the evaporation zone. The depositing rates locally were between 4 and 9.7 micrometers per minute, but could not be distributed over a larger surface, and the precipitated coatings contained to an appreciable extent metals which did not participate in the reaction.
In the coating of materials with hard substances on a large industrial scale, therefore, the method of magnetic-field-supported cathode sputtering has been used almost exclusively, but in that case rates of deposit of about 1 to 1.5 micrometers per minute cannot be exceeded substantially, if at all.
The invention is therefore addressed to the problem of improving a method of the kind described in the beginning so that coatings containing stoichiometrically reacted components of metals and gases will be obtained at high deposit rates, and enabling the method to be practiced on a large industrial scale under stable conditions of operation.